Spiral wound gaskets are used for sealing connections typically in high pressure, high temperature industrial processing applications. Spiral wound gaskets are formed by winding a continuous length of a thin narrow metal band in overlapping relation around a die so that the metal band spirals around. The overlapping layers of the band define the narrow radially-thick gasket having an inner diameter and an outer diameter for being received in a sealing application. The gaskets typically included a sealing material that wrapped the metal band. Previously, the sealing material was formed with strips of asbestos. The asbestos provided high temperature sealing capabilities for the spiral wound gasket. Asbestos however fell from favor and the industry began using graphite sheet material.
Graphite has long been recognized as a material which exhibits superior performance characteristics for sealing applications requiring gaskets and packing. These characteristics include high thermal stability, low thermal conductivity, natural lubricity, resistance to chemical degradation, conformability, and resilience.
Graphite has typically been provided in the form of calendared sheets made with expanded intercalated flake graphite worms. Intercalated flake graphite is made by treating natural or synthetic graphite flakes with an intercalating agent such as fuming nitric acid, fuming sulfuric acid, or mixtures of concentrated nitric acid and sulfuric acid. The intercalated flake graphite is then expanded at high temperatures to form a low-density, worm-like form of particulate graphite having typically an 80–100 fold increase in size over the flake raw material. U.S. Pat. No. 3,404,061 describes the production of intercalated flake graphite as an intermediate step in the production of expanded intercalated graphite. Expanded intercalated graphite worms have thin structural wall and are light-weight, puffy, airy, and elongated bodies.
These characteristics of expanded intercalated graphite worms lead to exceedingly difficult volumetric, handling, and use problems. A significantly large volume of the worms is required to produce a relatively thin layer of gasket material. There is an approximate 100 to 1 ratio between the volume of expanded worms and compressed worms. The worms being extremely lightweight, are difficult to handle. The slightest air current quickly stirs up the worms.
Because of these characteristics, expanded intercalated graphite worms are calendared to produce sheets of graphite. Calendared graphite is commercially available as GRAFOIL brand sheets. The calendared sheets known as “paper” have uniform density and uniform thickness. The calendared sheets are generally available in several standard thickness and densities.
The sheet may be die-cut to form a gasket or cut into strips. To provide increased tensile strength, a layer of mylar adhesive is applied to one surface of the sheet. The mylar allows the cut sheet to be applied to a substrate, such as an annular metal disk. Strips of the calendared sheet are also applied to the metal band and wrapped in the above-described spiral to form a graphite-based spiral wound gasket.
Gaskets manufactured with calendared graphite sheet typically are used for sealing purposes in high pressure, high temperature fluid flow applications. While the use of graphite gaskets perform sealing functions and have advantages over the use of asbestos-based gaskets, there were disadvantages as well. Cut calendared graphite sheet particularly provides open edges which is susceptible to possible high pressure attack from the fluids being sealed by the gasket.
The graphite based spiral wound gasket had higher minimum seat. The term “minimum seat” refers to the loading required in order to set the gasket for sealing. The higher minimum seat therefore required an increased load over that required for asbestos based spiral wound gaskets. Higher loads led to a bolt load retention problem. As the connection sealed with the spiral wound gasket experienced changes in temperature and pressure, the connection would flex. The loading would change and loosen in response to these fluctuations. Periodically, the loads imposed by the fasteners on the connection needed to be checked and reset. Two problems arose from the use of connections improperly loaded for spiral wound graphite gaskets. These problems were bent flanges and a condition known as interbuckling. The term “interbuckling” refers to a side wall of the gasket collapsing into the interior of the connection.
In response to these problems, graphite-jacketed wire mesh core packing was developed. This type of packing provided a low minimum seat with a metal mesh core for rigidity and strength. One drawback however to the use of such sealing material was the large range of flanges, sizes and pressures necessary for proper service. While the graphite jacketed metal core gasket material was directed towards application for spiral wound gaskets, this type of sealing material generally replaced flexible paper gaskets.
In order to resolve the interbuckling problem, gasket manufacturers added an inner guide ring. The inner guide ring supported the lateral side wall of the spiral wound gasket and restricted interbuckling of the wall. This solution however increases the cost of the gasket significantly. The use of inner guide rings to prevent interbuckling is generally limited to service applications requiring an exotic metal due to the corrosive nature of the material being sealed.
In addition, the recognized temperature range available for graphite-formed spiral wound gaskets has been decreased as experience developed using such gaskets. When initially developed, it was believed that graphite-formed spiral wound gaskets would be suitable for use up to 5000° F. Through experience, it is now believed that the suitable temperature range for graphite-formed spiral wound gaskets is limited to about 650° F. To increase the temperature range, the graphite-formed spiral wound gaskets were treated in a acid bath, such as phosphoric acid. Such treated gaskets are believed suitable for use up to about 850° F.
Accordingly, there is a need in the art for an improved spiral wound gasket and method of manufacture which overcomes the temperature, loading, and performance limitations of the present spiral wound gaskets. It to such that the present invention is directed.